/Engine/Source/Runtime/ApplicationCore/Public/GenericPlatform/GenericPlatformInputDeviceMapper.h
Returns the platform user id that is being used for unmapped input devices. Will be PLATFORMUSERID_NONE if platform does not support this (this is the default behavior)
[FPlatformUserId](API\Runtime\Core\Misc\FPlatformUserId) GetUserForUnpairedInputDevices() const